The women were 18-85 years old. They were diagnosed at the Mayo Clinic in Rochester, Minn., with benign breast disease between 1967 and 1991.

Over about 15 years, the group had 707 breast cancers. That's more than what's typical among women in the general public, writes Hartmann, who works in the Mayo Clinic's medical oncology department.

Family history of breast cancer was important. So was the type of benign breast lump.

Having a family history of breast cancer raised the odds of getting breast cancer. Women without a family history of breast cancer and benign lumps (those not characterized by multiplying cells) were not more likely to get breast cancer.